1. Field of the Invention
The present invention relates in general to a control apparatus for controlling an automatic transmission of a vehicle, and more particularly to techniques associated with a fail-safe arrangement enabling the vehicle to run in the event of operational abnormality in controller valves which are operable to selectively establish a plurality of gear positions.
2. Discussion of Related Art
In a vehicle automatic transmission in which a plurality of hydraulically operated frictional coupling devices are selectively engaged and released to selectively establish a plurality of gear positions having respective different speed ratios, there is provided a control apparatus for controlling a hydraulic pressure of each of the hydraulically operated frictional coupling devices. As such a control apparatus, there is known an apparatus which includes a plurality of solenoid valves serving as coupling-device controller valves operable to control flow of a pressurized working fluid that is to be supplied to the respective hydraulically operated frictional coupling devices, so as to selectively engage and release the frictional coupling devices. JP-H-09-303545A discloses a fail-safe arrangement which is to be employed in the control apparatus. In the disclosed arrangement, a fail-safe valve is provided between the coupling-device controller valves and the frictional coupling devices. When the controller valves are brought into a pressure-output failure state, due to electrical connector disconnection, wire breakage or other factors, in which the valve becomes incapable of applying hydraulic pressure to the frictional coupling devices, the fail-safe valve is controlled to establish an emergency fluid passage allowing the application of hydraulic pressure therethrough to the frictional coupling devices for establishing a predetermined one of the plurality of gear positions as an emergency gear position, so as to enable the vehicle to keep running.
However, in the conventional control apparatus with the above-described fail-safe arrangement in which the fail-safe valve is provided between the coupling-device controller valves and the frictional coupling devices, even where the controller valves do not suffer from the pressure output failure, the hydraulic pressure is applied to the frictional coupling devices via the fail-safe valve. That is, this arrangement always requires a long fluid channel, irrespective of whether the pressure output failure of the controller valves is present or not, making it difficult to enable the coupling devices to be accurately controlled in quick response to the coupling-device controller valves.